1. Field of the Invention
This invention relates to an aqueous suspension polymerization process for producing polymeric, esp.,polyvinyl chloride and PVDF, products with improved color, the process utilizing combinations of an N substituted hydroxylamine shortstopper with certain polymerization initiators.
2. Description of the Prior Art
In the manufacture of polyvinyl chloride, it is important commercially to obtain the best yield and quality at the lowest cost. Both initiator and shortstop can be significant factors. Ideally, PVC should be produced with effective, cheap, non-toxic, non-yellowing shortstopper and initiator.
Di 2-ethylhexylperoxydicarbonate and di(sec-butyl)peroxydicarbonate (marketed and designated as Lup 223 and Lup 225 respectively) are inexpensive initiators which work well but produce PVC with poor color.
The most commonly used short-stoppers for the aqueous suspension polymerization of vinyl chloride are ATSC (acetone thiosemicarbazone), bisphenol A (4,4xe2x80x2-isopropylidenediphenol) and Irganox(copyright) 245 (2,4-dimethyl-6-sec-hexadecylphenol), alone or as a mixture with Irganox(copyright) 1076 [octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate]. Irganox(copyright) 1141 (hereinafter IGX 1141) is a commercial mixture of 80 parts by weight of Irganox(copyright) 245 and of 20 parts by weight of Irganox(copyright) 1076. However, these short-stoppers all have some drawback: poor water solubility, slow to stop polymerization, toxicity/handling issues.
N,N-diethylhydroxylamine (DEHA), a representative substituted hydroxylamine compound, has been used in PVC production as a vinyl chloride stabilizer, as a xe2x80x9ckillerxe2x80x9d to stop runaway reactions, as an antiscalant, and when added prior to initiation at low levels, to modify PVC density (sometimes referred to as xe2x80x9cstunningxe2x80x9d the initiator) or to prevent unwanted polymerization during the xe2x80x9cwarm-upxe2x80x9d period.
Hydroxylamine compounds are also known in the industry as PVC shortstoppers; however, most tests indicated that DEHA caused yellowing of the polymer.
U.S. Pat. No. 3,222,334, (Dec. 7, 1965) discloses in the specification that N,N-dialkylhydroxylamines may be used to stop emulsion polymerization of vinyl chloride. However, no examples are given, and no advantage is noted. The reference suggested that dialkylhydroxylamines have a xe2x80x9cnon-toxic and non-discoloring naturexe2x80x9d, but this is in reference to use with various rubbers, and the distinction may be drawn between non-discoloring and stabilization of color; the former suggests non-involvement in color formation whereas the latter implies an interference in color-producing processes.
World Patent Application WO 98/51714 discloses the use of ethylhydroxylamine as a shortstopping agent for free radical polymerization. However, novelty is claimed through the allegedly xe2x80x9csurprising non-volatilityxe2x80x9d of ethylhydroxylamine, as well as its low corrosivity and water solubility. However this patent, while claiming use for all free radical polymerizations, discusses only emulsion polymerization of rubbers, and all the examples address comparisons related to such processes.
U.S. Pat. No. 4,749,757 includes the use of DEHA (and other polymerization inhibitors)in conjunction with PVA (a suspension agent)to simultaneously increase the density and porosity of PVC by maintaining a low concentration (5-10 ppm preferred)in the reactor. However, DEHA is not disclosed as a shortstopper for PVC.
In JP 02235912, Kanolo et al describe a xe2x80x9creaction product of DEHA, cyclopentadiene, and pyrogallolxe2x80x9d which when applied to reactor walls prevents the formation of scale. This relates to a known application which exploits the properties of hydroxylamines as part of a coating to impart antiscaling properties to the walls of PVC reactors.
It is also known in the art to use DEHA as a xe2x80x9ckillxe2x80x9d agent, to halt runaway vinyl chloride polymerization. Due to DEHA""s excellent solubility and thus rapid dispersability in water, it is effective in halting vinyl chloride polymerization when, for example, an electrical failure occurs and mechanical stirring ceases.
In the present invention, it has been discovered that combinations of substituted hydroxylamine shortstopper with certain organic initiators provide excellent polymerization kinetics and a rapid, efficient shortstopping, resulting in high yields of high-quality polymer at low cost.
When vinyl chloride resin derived from the described process is compounded, the thermal stability and color of the product is sometimes found to be improved over resin which has been made with no shortstopper.
Another aspect of this invention relates to improving the color of extruded PVC by adding a substituted hydroxylamine to the heat stabilizer.
Another aspect of the invention relates to the process which provides excellent polymerization kinetics and a rapid, efficient shortstopping, resulting in high yields of high-quality polymer at low cost by the use of combinations of alkyl substituted hydroxylamine shortstopper with dialkyl peroxydicarbonates, peroxy-tert-alkanoates and diacyl peroxides initiators.
In another aspect of the invention for the aqueous suspension polymerization of vinyl chloride, alone or as a mixture with another vinyl monomer, the polymerization initiator comprises at least one compound chosen from dialkyl peroxydicarbonates, peroxy-tert-alkanoates and diacyl peroxides and use is made, as agent for halting the polymerization, of a mono- or dialkylhydroxylamine, each alkyl radical of which comprises from 1 to 4 carbon atoms. The resins thus obtained exhibit excellent whiteness.
A final aspect of the invention relates to improved uptake of plasticizer which occurs when shortstopping is effected with mono- or dialkylhydroxylamines.